Pulse oximetry utilizes an electro-optical sensor attached to a patient and a monitor that measures the electro-optical signals and calculates patient oxygenation. Because of its simplicity, reliability, and ability to quickly report changes in patient oxygenation status, pulse oximetry measurement has been employed for the past twenty years as a standard of care in critical care monitoring of patients.
External or non-invasive pulse oximetry devices typically utilize light transmittance or reflectance technology incorporating a light source and a detector (or sensor) operatively attached to an individual. Light emitted from the light source is passed through the individual's tissue wherein a portion thereof is received by the sensor and analyzed to determine the blood saturation level of the tissue.
Nearly all pulse oximeter devices employ two LED light sources (red and infrared) and a silicon photodiode for the measurement in the sensor. Various sensor topologies have been utilized in the prior art, including finger type “clip” sensors, disposable sensors constructed in tape laminates, elastomer based tube sensors, and multi-site “Y” or “flag” shaped sensors. All these sensors use the above electro-optical elements in combination with cable and connector configurations that allow connection to the appropriate monitor type.
In practice, LED light sources are sequentially pulsed and the light enters a patient site that contains an arterial bed, e.g., finger or toe. After passing through the site, a portion of the light is received by a photodetector and converted into a current that may be on the order of several to tens of microamperes in magnitude. This relatively small electrical signal is transmitted via a sensor cable (and an extension cable, if necessary) to a monitor, where it is amplified, measured, and interpreted according to the monitor's measurement algorithm(s).
Because oximetry generally relies upon detection and measurement of minute current signals, effects related to signal-to-noise limitations should be considered. The operational environment for oximeter devices typically includes other electronic devices nearby, e.g., surgical equipment. Some common techniques used to address signal-to-noise performance include utilization of shielded cables and detectors, and a method of pulsing of the LED's at a high current (on the order of 40 to 400 mA) for a short duty cycle to temporarily boost the light output and detector signal. However, such techniques necessarily will not overcome all the signal-to-noise limitations inherent in the measurement of minute currents. Additionally, LED intensity levels may gradually decline over the life of the sensor and may lead to an eventual failure of providing usable signal levels.
A typical reusable finger clip SpO2 sensor is comprised of a hinged housing made of rigid plastic, which holds flexible pads that contact a patient's skin, e.g., finger. The pads contain optical elements preferably covered with a transparent material, e.g., layer, pane, window; that facilitate measurements through the patient's finger. One particular finger clip sensor, the Nellcor DS-100A, has pads made from a white silicone material, and a clip configuration that is open and permits ambient light to enter the finger from the sides. Ambient light is therefore added to the detector baseline signal, and may in some cases affect the signal measurement quality.
The white color of the pads is generally highly reflective in both the red and infrared wavelengths present in the sensor, and assists the reflectance of several internal pathways. As such, utilization of the white pads facilitates the throughput of the generated light signals and generally provides a more efficient transfer of light from the LED to the detector. Referring to FIG. 2, the diagram is illustrative of this feature—showing light exiting the light source and passing through the finger via multiple transmissive, reflective, and scattering paths and entering the detector.
While the complexity of the finger and white pad as reflective and scattering media makes exact characterization of the light paths difficult, it can be readily demonstrated that a white pad facilitates more throughput of measurable light than the equivalent optics in a darker pad. As can also be seen, ambient light may also reach the detector by travelling through the finger or reaching the pad. Therefore the white pad is efficient in light transmission, but remains susceptible to ambient light interference.
Another sensor, the BCI 3044 Finger Clip sensor, utilizes a rigid housing that extends over the pads to reduce the influence of ambient light, as well as finger-contacting gray colored pads. Another sensor, the HP M1191A, utilizes a sensor housing constructed primarily of a gray rubber tube that surrounds the finger. Although both of these sensors have the advantage of reducing the admittance of far less ambient light, throughput efficiency of the light signals is reduced due to the darker pad.
Dark pads (and coverings) reduce susceptibility to ambient light, but are less desirable for directing the light from the transmitters (LED's) to the receiver (photodiode) because of less efficient utilization of internal reflectances. In practice, dark pads are less desirable because signal detector current levels may be reduced by a factor of 2-3 as compared to light colored pads.
In summary, utilization of the sensors described above requires an unfavourable trade-off between signal quantity and susceptibility to ambient light interference. That is, optimal detection and measurement parameters are not provided by use of either monocolor light (white) or dark (gray, black) pads.
The present invention is intended to address these as well as other shortcomings in the prior art.